This invention relates to a process for the preparation of electrophotographic developer materials and more specifically to a process for altering the surface of a conductive toner in order to substantially improve the fusing characteristics of such a toner. In one embodiment, magnetic toner materials may be utilized which are applied from a magnetic brush to the electrostatic latent image without the use of a carrier material.
In an electrophotographic process such as a xerographic process, a plate comprising a conductive backing upon which is placed a photoconductive insulating surface can be uniformly charged and thereafter the photoconductor surface exposed to a light image of the original to be reproduced. The photoconductive surface is prepared in such a manner so as to cause it to become conductive under the influence of the light image in order that the electrostatic charge formed thereon can be dissipated selectively to produce what is developed by means of a variety of pigmented resin material specifically made for this purpose such as toners. The toner material used is electrostatically attracted to the latent image areas on the plate in proportion to the charge concentration contained thereon; thus, for areas of high charge concentration there is a correspondingly high toner density. Subsequently, the developed image can be transferred to a final support material such as paper and fixed thereto for a permanent record or copy of the original.
Several methods are known for applying the electroscopic particles to the electrostatic latent image to be developed including for example the development method referred to as cascade development as described in Wise U.S. Pat. No. 2,618,552, a magnetic brush process as described in U.S. Pat. Nos. 2,874,063; 3,251,706; and 3,357,402, a powder cloud development as described in C. F. Carlson's U.S. Pat. No. 2,221,776, and a touchdown development as described in R. W. Gundlach's U.S. Pat. No. 3,166,432.
There has also been suggested systems for developing images and particularly systems for magnetic development wherein a carrier material is not used. For example, in Wilson U.S. Pat. No. 2,846,333 there is disclosed the use of magnetic brush to apply toner particles formed of ferrite and resin material to develop electrostatic latent image. Another process utilizing magnetic toner involves development with a magnetic toner of magnetic images carried on a surface such as a magnetic tape as described in U.S. Pat. No. 3,804,511, one of the disadvantages of such a process being that there is a large amount of magnetite present which results in a toner image which is difficult to fuse particularly at a high rate or at normal fusing temperatures. Additionally, there is described in Kotz U.S. Pat. No. 3,909,258 the development of magnetic images without the use of carriers wherein an electrostatic development process utilizing a magnetic brush without carrier is described. A toner suitable for use in the Kotz process is described in U.S. Pat. No. 3,639,245 to Nelson wherein a dry powder having specific electric conductivity is disclosed. The toner of Nelson is generally formed by melt blending magnetite with the resin and subsequently pulverizing the resulting material to a small particle size and subsequently made conductive by coating the toner with carbon black using heat spheroidization. The resulting toner material is rather conductive due to the layer of surface carbon black and because of this higher fusing temperatures, especially when a toner is prepared in accordance with the process as described in Nelson wherein the toner is fused to zinc oxide paper which paper also is the photoreceptor, are needed in order to obtain substantially complete fusing or if lower fusing temperatures are employed inefficient fusing is obtained. The process of the present invention is particularly useful for treating toners of the Nelson patent, U.S. Pat. No. 3,639,245.
Fixing of the images can be accomplished in a number of techniques including for example, vapor fixing, heat fixing, pressure fixing, or combinations thereof as described for example in U.S. Pat. No. 3,539,161. These techniques of fixing do suffer from some deficiencies which render their use either impractical or difficult for specific electrostatographic applications; thus, for example, it has been found rather difficult to construct an entirely satisfactory heat fuser which has short warm-up time, high efficiency and ease of control. Another problem generally associated with heat fusers is that the burners scorch the support material, for example paper. Similar problems exist with pressure fixing methods whether used with heat or without heat and more particularly the problems involved here include, for example, image offsetting, resolution degradation, and further it cannot consistently produce a good permanent type of fix. The amount of energy needed to cause the fusing is important and generally the higher the amount of energy needed or the higher the temperature is utilized for causing fusing correspondingly the more difficulties are encountered and the lowering of the fusing temperature by several degrees up to 10 degrees has a beneficial effect on the whole system.
Processes and materials which result in lower fusing temperatures when using magnetic toners without carriers are described in copending application U.S. Ser. No. 813,602 filed on July 7, 1977 in the name of Xerox Corporation. Generally in th present invention, the process as described in the copending application is employed but for a more specific purpose, that is for modifying toners containing conductive materials such as surface carbon black, as described in U.S. Pat. No. 3,639,245 in order to lower the fusing temperature.